Why Denver Has Extreme Radon Levels
Radon is a radioactive gas produced by the natural decay of uranium in soil and rock. Denver's Front Range geology contains uranium-bearing granite formations that produce radon continuously. The gas seeps up through soil and enters buildings through cracks, gaps, and openings in foundations.
The EPA designates Denver and most of Colorado as Zone 1, the highest radon potential category. This means the predicted average indoor radon level exceeds 4 pCi/L—the level at which EPA recommends taking action.
How Radon Enters Homes
Radon enters buildings through any opening in contact with soil: cracks in foundations, gaps around pipes, basement floor drains, sump pits, and porous concrete. The gas is invisible and odorless, making testing the only way to detect it.
Homes with basements or slab-on-grade construction have highest exposure potential. Crawlspace homes can also have elevated levels if air from the crawlspace enters living areas.
Health Implications
Radon is the second leading cause of lung cancer after smoking and the leading cause among non-smokers. The EPA estimates radon causes about 21,000 lung cancer deaths annually in the United States. Risk increases with exposure level and duration.
Radon Testing During Home Inspection
Standard home inspections in Denver routinely include radon testing, either as part of the base inspection or as an add-on service. Testing takes 48-96 hours using continuous monitoring devices or charcoal canisters.
Testing Conditions
Accurate testing requires closed-building conditions: windows and exterior doors closed for 12 hours before testing begins and throughout the test period. Normal entry and exit is fine, but extended open windows or doors invalidate results. The testing device is placed in the lowest livable area of the home.
Interpreting Results
The EPA action level is 4.0 pCi/L. Results below 2.0 pCi/L are considered good. Results between 2.0-4.0 pCi/L may warrant mitigation. Results above 4.0 pCi/L warrant action. Results above 10 pCi/L indicate significant exposure.
For context, outdoor air averages about 0.4 pCi/L. The average American home tests at 1.3 pCi/L. James's reading of 14.2 pCi/L was roughly 35 times outdoor levels.
What Happens After High Results
When radon tests high during a home purchase, buyers typically request the seller install a mitigation system or provide a credit for installation. Given how common elevated radon is in Denver, sellers generally expect and accommodate these requests.
Mitigation System Basics
The most common mitigation method is sub-slab depressurization. A pipe is inserted through the basement floor into the gravel or soil beneath. A fan pulls radon-laden air up and exhausts it above the roofline, where it disperses harmlessly. The system runs continuously.
Installation typically costs $800-1,500 in Denver. The fan runs 24/7 with annual electricity costs around $30-50. Systems last 15-25 years with occasional fan replacement.
Effectiveness of Mitigation
Properly installed systems reduce radon levels by 80-99%. James's home went from 14.2 to 1.8 pCi/L after mitigation—an 87% reduction. Post-mitigation testing confirms the system is working.
Homes With Existing Systems
Many Denver homes already have radon mitigation systems. When buying a home with an existing system, verify it's operational (fan running, no disconnections) and request post-installation test results. The system should be tested every two years to ensure continued effectiveness.
Radon in New Construction
New homes in Denver are typically built with radon-resistant features: a gravel layer beneath the slab, plastic sheeting as a vapor barrier, sealed penetrations, and a vertical PVC pipe from beneath the slab that can be connected to a fan if needed.
However, these passive features don't guarantee low radon levels. James's seven-year-old home had radon-resistant construction but still tested extremely high. Testing is essential regardless of home age.
Converting Passive to Active
If a new home with radon-resistant features tests high, converting to an active system is straightforward. Adding a fan to the existing pipe typically costs $300-500—much less than installing a complete system in an older home without the rough-in.
What James Would Tell Every Denver Buyer
James now tells everyone buying in Denver: test for radon during inspection, and test again after you move in. His seven years of unknowing exposure bothers him, though he takes comfort in knowing his lifetime risk increase is relatively modest.
More practically, he advises treating radon like any other serious inspection finding. High levels aren't a reason to walk away from an otherwise good house—mitigation is effective and affordable. But understanding the situation before closing lets you negotiate appropriately and ensures mitigation happens before you move in.
Denver's radon situation is a fact of the geography. Like the thin air and the dry climate, it's something residents learn to manage. Testing and mitigation make it manageable.